ANCHORAGE, Alaska — When Royal Dutch Shell's oil drilling rig Kulluk and tow ship, the Aiviq, pulled out of Dutch Harbor the afternoon of Dec. 21 for a long, slow trip to Seattle, Shell says it was relying on its consultant's weather forecast to ensure crews -- and prized vessels -- arrived safely.

"The forecast was within the operational thresholds to begin the journey," Shell spokesman Curtis Smith said Friday afternoon. Without that window, the Shell vessels would not have left Dutch Harbor, he said.

But less than a week into the trip, as the Shell-contracted Aiviq hauled the Kulluk across the Gulf of Alaska, one of the region's notorious storms hit with a vengeance. Over five days, multiple vessels that came to help lost towlines to the thrashing Kulluk. The rig, by then unmanned, grounded Dec. 31 just off Sitkalidak Island, south of Kodiak.

As inquiries by the Coast Guard, the Obama administration and U.S. Sen. Mark Begich try to get to the bottom of the serial failures, questions are swirling about whether the Aiviq was capable of towing the Kulluk during the type of storm that regularly sweeps the Gulf of Alaska, and whether government oversight was adequate.

Shell's assessment of the forecast as favorable was seriously flawed, says Cliff Mass, a University of Washington professor of atmospheric sciences whose specialty is weather forecasting.

The North Pacific and Gulf of Alaska are among the stormiest places on Earth, he said. Forecasts more than four or five days out quickly become unreliable. And even the short-term forecast on Dec. 21 pointed to rough weather coming, Mass said.

"They were showing storms. The models were predicting stuff," Mass said. "The whole logic fails, in a number of ways." His models from Dec. 21 showed storm-generating low-pressure systems already forming in the Aleutians.

Mass was so irritated by Shell's assertions that he wrote a lengthy post disputing them on his weather blog.

"Shell Oil made a misguided and poorly informed decision," he concluded.

The National Weather Service on Dec. 21 predicted 35 mph winds and 12-foot seas by Dec. 25 in the area where the Kulluk eventually grounded.

Data from buoys in the Gulf of Alaska show that gale force winds -- 39 mph or stronger -- occur 15 percent of the time during December and January, according to the weather service. Seas of 17 feet or higher occur about 20 percent of the time. The Gulf experiences hurricane-force storms with winds topping 74 mph two or three times a year, the weather service said.

Asked to provide the forecast Shell used, Smith initially referred the Daily News to the company's consultant, ImpactWeather, located in Houston, Texas. But in an email Friday he said the private forecaster provided wind and wave predictions similar to the National Weather Service's.

The severity and duration of the coming storm wasn't clear until Dec. 24, a day before it hit, Smith said. "The storm did not materialize undetected," he wrote.

ImpactWeather cannot discuss services for a particular client, said Chris Wolf, the company's marketing director, but "in meteorological science, there is no certainty assigned to any of it."

Wolf said she hadn't seen the forecast for Shell. "That's a completely different part of the company and they are looking into it," she said. The company wants to examine what happened too, she said.

Mass attended the annual conference of the American Meteorological Society in Austin, Texas, last week. He saw that ImpactWeather had a table, "so I went over there and I said, 'Did you give a two-week weather forecast (to Shell)?' " Mass said. "And they started laughing at me."

No one could produce a reliable two-week forecast for that area, Mass said.

Not Shell, the Coast Guard nor a command team that has managed the Kulluk situation leading up to and after the grounding would provide a copy of the towing plan prepared for the trip, or any information it contained about the maximum waves and winds the tow setup could be expected to handle.

"That specific information is currently being gathered for the purpose of internal and external investigations -- the outcome of which (external investigations) will be made public in the future," Smith said. (Shell has not said whether it will make its own internal investigation public.)

The towing plan was reviewed by a Coast Guard marine inspector in Anchorage after Shell, or one of its contractors, voluntarily submitted it, said Petty Officer David Mosley.

While the Coast Guard didn't formally approve the plan -- that wasn't required -- the specially designed Aiviq, with four engines, seemed more than capable, "even in very extreme conditions," said Capt. Paul Mehler, the Coast Guard commander in Anchorage.

At a community meeting in Kodiak on Wednesday, retired school teacher and former fisherman George Griffing asked Shell officials why they seemed surprised by the December storm and why the company relied on a forecast that was going to be outdated long before the Kulluk made it across the Gulf of Alaska.

The Aiviq's top speed during the tow was about 4.5 mph, a rowboat pace. It alone was supposed to tow the Kulluk, as it had done last summer on the journey north from Seattle.

If the storm that grounded the Kulluk was extreme, Griffing said, then the Gulf is "constantly extreme" in winter. Why leave Alaska in winter, with the predictable challenges of darkness and foul weather, he asked.

Shell had originally planned for the Kulluk to winter over in Dutch Harbor in a special $1 million custom dock.

Before the tow went bad, Shell spokesman Smith said the timing of the Kulluk's departure was motivated in part by the possibility that the company might incur a multimillion-dollar state tax liability if the rig was still in Alaska on Jan. 1. After the grounding, he said he misspoke and that while Shell was aware of the tax issue, the decision to leave port came after inspections revealed the extent of needed off-season maintenance, including replacement of the rig's cranes. Shipyard work is difficult in Dutch Harbor, he said, because of challenging logistics, unpredictable flying weather and a limited work force.

At Wednesday's meeting, Shell's Alaska operations manager Sean Churchfield said a lot of work went into designing a solid tow plan, including weather forecasting, routing and equipment capabilities. Shell expects the investigations to explain the multiple failures of towline equipment, he said, as well as why all four of the Aiviq's engines lost power and had to be repaired at sea during the storm.

A warranty surveyor who works for Shell and is expert in marine operations had to approve the tow plan; the surveyor inspected the rig, the fastenings and tow equipment, Smith said in an email. A tow master, who was aboard the Kulluk until it was evacuated, also had to sign off. Several high-level department heads at Shell approved the plan. So did the captain of the Aiviq, Smith said.

"It's quite a process," he said.

Shell was bringing the Kulluk back from the Beaufort Sea, off Alaska's north coast. The rig was used to drill the top part of a single well during the short 2012 drilling season, Shell's first in the Alaska Arctic in two decades. Environmentalists argue that troubles with the Kulluk and Shell's other drilling rig, the Noble Discoverer, show the company is not adequately prepared.

On Thursday, the Environmental Protection Agency said both drilling rigs had multiple violations of air emissions permits. Shell says it welcomes various inquiries but remains confident in its program.

Shell says it has invested some $292 million in upgrades to the 30-year-old Kulluk, which it bought in 2005, but the company hasn't been willing to provide an estimate of the rig's value. Shell has put nearly $5 billion into its Alaska drilling program, counting the money spent on oil leases.

"This incident was not caused by weather," Smith said in an email Friday. "It was compounded by it. It appears that a sequence of unlikely events occurred over a short period of time -- underscored by the complete loss of power to the propulsion engines on the Aiviq."

SHIP ON STEROIDS

A subsidiary of Edison Chouest Offshore, a Louisiana-based private maritime company, built the 360-foot-long, $200 million Aiviq for Shell as a multi-purpose Arctic ship. Edison Chouest owns it and contracts with Shell to operate it.

The Aiviq can break ice. It can help recover spilled oil. It can anchor drilling rigs to the sea floor. It has spacious crew quarters. And it can tow.

Cmdr. Jim Rocco of the Coast Guard's Outer Continental Shelf National Center of Expertise in Morgan City, La., said the Aiviq is the biggest vessel of its type in the country.

"This is what we might consider an offshore supply vessel on steroids. Because it does have all the power it has and just its sheer size," Rocco said. "I remember the first time I saw that vessel. I thought, 'Did somebody just cut off the front of a cruise ship and make that the Aiviq?' "

The Kulluk situation has drawn hundreds of posts on the popular maritime blog, gCaptain. Some commenters have wondered whether two small, traditional tugs might have worked better than a massive, challenging-to-maneuver ship like the Aiviq.

Rocco said the Aiviq is well designed to hold and move the Kulluk, which weighs 18,681 tons empty. Support vessels like the Aiviq are commonly used for towing mobile oil rigs, he said. But two smaller tugs might maneuver better if the crews were close to shore or trying to get to a safe harbor, an idea that was considered shortly before the Kulluk grounded.

Repeated calls by the Daily News to Edison Chouest to discuss the Aiviq were not returned.

The firm's president, Gary Chouest, is a heavyweight political campaign donor. Chouest, his employees and close family members are the top contributors to U.S. Sen. Lisa Murkowski and Rep. Don Young, and among the top donors to Sen. Mark Begich, according to the Center for Responsive Politics.

A TURNING SAUCER

In a call from the Alex Haley, a Coast Guard cutter helping the Kulluk and Aiviq almost from the start, Lt. Dave Gilbert compared the 160-foot-tall derrick at the center of the Kulluk to "a sail that was pulling them back."

Of at least five vessels that had a towline on the Kulluk at various times before the grounding, only the tugboat Alert, owned by Crowley Marine Services, never lost its tether.

Yet in the pummeling Dec. 31 storm, even the Alert was pulled backward by the Kulluk. Four miles from land, its crew was ordered to let its line go to avoid jeopardizing their safety and their vessel.

Towing the round Kulluk is nothing like towing a big oil tanker, which the Alert, as a Prince William Sound tanker escort vessel, is designed to do, said Charlie Nalen, Crowley Marine Services vice president of operations for Valdez.

"This big saucer is turning. It's an ungainly structure," Nalen said. In contrast, a tanker will follow along behind a tug.

The Alert's captain during the ordeal, Rodney Layton, is Crowley's most veteran skipper with 30-plus years of experience. At a crew changeover this week in Seward, where a Shell executive personally thanked the crew, Layton said the December storm was the worst he has ever seen. Crashing waves 50 feet tall damaged the tug's lights. The crew worked in pitching seas, darkness and cold; they said they were glad for their training in emergency drills in Prince William Sound, Nalen said.

The towline to the Kulluk was maybe 300 feet long, a football field away, the captain said during the debriefing, as Nalen recalled.

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